## College Physics (7th Edition)

a). (4) $(alpha-beta)$ b). $20^{\circ}$
a). Angle BAO = $90^{\circ}-beta$ Angle ABO = $180^{\circ}-[(90^{\circ}-beta)+alpha]=90^{\circ}+beta-alpha$ Thus, angle of incidence of beam from the second mirror = $90^{\circ}-(90^{\circ}+beta-alpha)=(alpha-beta)$ Also, angle of reflection = angle of incidence. Option (4) is the answer. b). Angle of reflection of the beam from the second mirror = $(alpha-beta)$ $60^{\circ}-40^{\circ}=20^{\circ}$